As somebody who worked in a Tesco petrol station during my misbegotten uni years, you should be aware of the following:
1) Even if the supermarket and its garage isn’t 24 hours at the moment, doesn’t mean it won’t be in the future. The Tesco I worked at had 7am-10pm for the main store, and 6am-midnight for the petrol station. It then changed to 24 hours for both. After a couple of years, the main store went back to its previous hours, but the garage remained 24 hours.
2) I could be wrong, but I don’t think they can have a completely unmanned petrol station, for safety reasons. They certainly couldn’t back in the day (you had to have one attendant for every 6 pumps).
3) Tanker deliveries can be very early in the morning – again, unless regulations have changed, there needs to be somebody from the garage to take delivery of the fuel in addition to the tanker driver, to ensure that there are no mistakes (which doesn’t always work, I had a colleague who didn’t pay attention and ended up costing Tesco over £65k in one day. Naughty boy.)
4) If the garage is going to have a reasonably stocked supply of snacks AND is 24 hour, there will be loud drunken bellends on a Friday and Saturday night. Or every night, if you live somewhere with a student population.